Morrison, K., Bury, S., Thompson, D., (2014). Higher trophic level prey does not represent a higher quality diet in a threatened seabird: implications for relating population dynamics to diet shifts inferred from stable isotopes. Marine Biology. 161:2243–2255. DOI: 10.1007/s00227-014-2502-y
Figuring out an optimal diet for ourselves is a lifelong affair, and for animals that have to hunt, it’s even harder! Wild animals can’t just help themselves to a pantry full of energy and nutrient dense prey, they have to factor in the abundance and ease of catch as well. The best wild diet maximizes the energy gained, net of expenditures in the shortest amount of time. An eastern rockhopper penguin’s (Eudyptes chrysocome filholi) menu consists of zooplankton such as krill, as well as cephalopods and fish. And since fish, a higher trophic level prey, are more energy dense than zooplankton or cephalopods, and more easily digested, they are scientifically considered higher quality prey.
Formally, the largest population of eastern rockhopper penguins, a subspecies of southern rockhopper penguins could be found on Campbell Island in New Zealand. However, between 1942 and 1984, the population drastically crashed by around 94% from an estimated 800,000 breeding pairs to 51,500. Breeding penguins fast for long periods of time as they take turns incubating and guarding their eggs and hatchlings, so having a higher body weight often means higher reproductive success.
Some scientists posited that nutritional stress resulting from a shift in the availability of prey from a higher trophic level prey to a lower one, due to warmer sea surface temperatures caused the significant decline.
Morrison et al., wanted to investigate if penguins consuming higher trophic level prey such as fish and cephalopods actually achieve a better body condition as inferred by body weight. In order to do so, samples of blood from 70 chicks, 55 adult females and 55 adult males of eastern rockhopper penguins were obtained over three years (2010-2012) at Penguin Bay, Campbell Island between October and January. Body weight was measured using a spring scale and blood samples were taken using a needle and syringe and stored in ethanol. The samples were then processed for stable isotope analysis (SIA) during which nitrogen and carbon were analyzed. Nitrogen isotopes evaluate the trophic level of prey. Fish and cephalopods have higher 15N:14N ratio, or δ15N when compared with zooplankton. If the blood sample yields a high δ15N, this means a larger proportion of the penguin’s diet consists of higher trophic level prey. Carbon isotopes can identify the environment of prey. Plankton closer to shore have a higher 13C:12C ratio, or δ13C, compared to further, pelagic plankton. If a blood sample yields a high δ13C, then the penguin is favouring prey closer to shore. Results of the samples were then compared with body mass in order to tease out what exactly is a high quality diet for eastern rockhoppers.
Turns out, higher trophic level prey does not translate to heavier chicks. δ15N and δ13C values were not related to chick body mass. Older chicks (24 days after hatching) were even heavier in 2011 when fed lower trophic prey compared to 2012. As for adults, there was not a significant relationship between body mass at arrival to the colony to δ15N (See figure on left). However, Female δ13C values were positively correlated with body mass meaning females were foraging for prey closer to the colony site before breeding. While there was no correlation between body mass and δ15N values in females during incubation forages, the results for male rockhoppers were surprising. Contrary to expectations, males with lower δ15N and δ13C values were heavier, meaning successful males actually consumed lower trophic level zooplankton near the nest area!
Shifting to a lower trophic level prey will not necessarily result in nutritional stress and population decline. This result should stand as a precaution for assuming this in other species. Shifting to a lower trophic level diet may actually represent a higher quality diet if more abundant. These surprising results inspire an alternative hypothesis; that rockhopper penguins are more likely to thrive when zooplankton prey is plentiful. Rockhoppers are increasingly competing against commercial fisheries for their prey. Breeding penguins are of greater concern since they fast for long periods of time and are even more susceptible to a decline in local prey since they are colony-based when hatching and rearing their chicks. This brings to light urgent conservation implications, especially since they are considered vulnerable to extinction under IUCN. Insight into an optimal diet for these goofy looking critters, can ultimately be used to keep them around.
I’m a past oceanbites writer, occasional editor and guest poster. I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working in marine conservation. In particular, I’m supporting an Indigenous-led initiative to safeguard the largest inland sea in the world (Hudson Bay & James Bay, Canada). I love weird ocean critters and *sigh…I really do enjoy long walks on the beach.